Conventional methods which employ a microorganism to produce Phe involve culturing the microorganism in a liquid nutrient medium or incubating it with a raw material substance in a reaction liquid to produce and accumulate Phe in the culture or in the reaction liquid and the Phe is then harvested.
The crystal morphology of Phe is known to include alpha-crystals (hereinafter referred to as Phe-alpha-crystals) and beta-crystals (monohydrate crystals--hereinafter referred to as Phe-beta-crystals). Regarding the shape of the crystals, Phe-alpha-crystals are tabular or flaky, while Phe-beta-crystals are extremely fine, needle-like crystals. In conventional production of Phe employing a microorganism, the crystals which precipitate out of the culture medium due to accumulation of Phe to a concentration higher than its supersaturated solubility in the medium are always Phe-beta-crystals if no measures are taken to control the crystal form. Precipitation of Phe-beta-crystals during cultivation causes the viscosity of the culture medium to increase and over-foaming of the culture which makes continuation of the cultivation difficult. If further continuation of the cultivation were attempted under these conditions, it would cause another problem, that of scattering of the culture medium out of the cultivation device and, as a result, the production of Phe would be substantially impossible thereafter. Therefore, a microorganism which does not produce and accumulate Phe in an amount which saturates the culture medium must be employed, or, when microorganisms having a high producing ability are cultured, control of the amount of the raw material supplied or control of the amount of the culture medium must be effected to limit the amount of Phe accumulated to an amount lower than the solubility thereof. For these reasons, the productivity of Phe by conventional methods is low. Therefore, development of a drastically improved method for reducing the cost of producing Phe by employing microorganisms has been desired.